1. Field of the Invention
The present invention relates to an improved audio signal switch. Audio switches are used widely in home and professional audio equipment such as amplifiers and home entertainment units.
2. Description of the Related Art
In current audio-visual (AV) systems, it is frequently desirable to be able to select one of a plurality of analog audio sources for connection to an amplifier and thence to one or more loudspeakers. Example sources include audio tape players, CD players, DVD players, video tape players, radios, mini-disc players, MP3 players, video game consoles and televisions. Many current AV systems include seven or more audio input connections.
A problem with such AV systems is the disparity in signal levels which are produced by different audio sources. For instance, the SCART specification allows audio signals to be 2rms(≡5.6Vpeak-peak), while a typical Video Analog source is only a maximum of 1.4Vpeak-peak.
Prior art solutions include use of a CMOS switch, i.e., a standard CMOS transmission gate including NMOS and PMOS transistors arranged in parallel. This configuration is limited in that the MOS operating voltage needs to be higher than the signal amplitude to be switched, thus limiting this solution to cases where the amplitude of the signal(s) to be switched does not exceed the value of VDD, which is typically 5V.
In order to overcome the problem of the above described arrangement, certain systems employ selective matched attenuation and amplification. The incoming analog signal is attenuated by a pre-determined amount pre-switching, and then selectively amplified post-switching to re-create the original level of the signal. This allows the signal which is actually switched to be within the operating range of the CMOS devices. However, aside from the extra components introduced by such a scheme, errors will be introduced by the signal dividers used, which are commonly resistors. Typical devices will introduce a 1% error due to component value mismatch. This error is introduced into both AC and DC values. The AC error does not generally matter, but the DC error will be different for each source and thus creates a signal discontinuity which presents itself as an audible commutation ‘pop’ upon switching. Such a ‘pop’ is not generally acceptable and requires further additional processing, such as offset cancellation and smoothing, to rectify the situation.
A solution to this problem uses bipolar transistors arranged in an emitter follower configuration. Since such devices do not use the gate of CMOS components, which is the main voltage limitation of sub-micron MOS devices, it is possible to use relatively high-voltage bipolar components in a sub-micron BiCMOS process.
FIG. 1 shows such a prior art attempt to solve the problem of audio source switching. The circuit is arranged to switch between two input audio sources 100, 105. Although only two input sources are shown for clarity, the skilled person will realize that appropriate modifications may be made to allow for switching between any number of input audio sources.
The input sources 100, 105 are connected to the base of respective transistors 125, 120. The collector of each transistor is connected to a voltage supply Vcc. The emitter of each transistor is connected to one terminal of a respective current source 110, 115. The other terminal of the current source is connected to a ground potential.
The emitter of each transistor 125, 120 is further connected to the base of a further respective transistor 140, 145. In addition to said connections between said emitter of transistors 125, 120 and said bases of said transistors 140, 145, there is a switch arrangement 130 which is operable to selectively connect said intermediate connection to Vcc. In the case of more than two input sources, the switch is arranged to selectively connect all unwanted inputs to Vcc. Whichever connection is selected by said switch dictates which input audio source 100, 105 is presented at the output 150 of the switch circuit.
The emitters of transistors 140, 145 are connected together to a first terminal of a current source 135. The other terminal of current source 135 is connected to Vcc. The collectors of transistors 140, 145 are connected to a ground potential. The Emitters of transistors 140, 145 are connected together to the output port 150.
Transistors 125, 120 are NPN (vertical structure), and in conjunction with current sources 110, 115 either transistor 125, 120 will operate as an emitter follower depending on the status of switch 130. The switch 130 ensures that the inoperative transistor remains inactive. Either one of transistors 140, 145 in conjunction with current source 135 also operate as an emitter follower, depending which transistor is active, which is again controlled by switch 130.
The inputs 100, 105 are configured to share a common biasing system so that there is a minimal DC offset between the inputs. The input signal is generally AC-linked.
In FIG. 1, with the switch in the position shown, input 105 is selected. The position of the switch has the effect of shorting the emitter of transistor 125 and the base of transistor 140 to Vcc, thus disabling input 100.
A problem with the circuit of FIG. 1 is that the vertical bipolar transistor structure is not able to accept a very high reverse voltage for the emitter-base junction, unlike the base-collector junction—the maximum VEB is lower than the maximum VCB. This means that the amplitude of the switched signal is limited, and renders the circuit of FIG. 1 unsuitable for analog audio signal amplitudes.
In an attempt to remedy the problems with the circuit of FIG. 1, the circuit of FIG. 2 has been proposed. It has the same basic structure as the circuit of FIG. 1, and similar reference numerals refer to similar components. It differs to the circuit of FIG. 1 in that there is a diode 255, 260 connected between the base and emitter of transistors 225 and 220 respectively. The effect of the diodes 255, 260 is to limit the reverse biasing of the transistors 225, 220, VBE to −Vd, where Vd is equal to the diode voltage drop.
However, this solution introduces a new problem in that the non-selected input 200 sees its voltage shifted by diode 255 to (Vcc−Vd). That has the effect that if the non-selected input is selected by switch 230, there will be a perturbation of the signal during restoration of the DC level, with additional current flowing from the forward biased diode. This perturbation occurs during signal selection and has an adverse effect on audio signal quality.
Therefore, there is a specific problem with the amplitude levels of audio signals in that they exist over a wide range, and this problem is further exacerbated by problems introduced by the trend towards reduced operating voltages for integrated circuits, which makes it problematic to interface with signals at levels in excess of supply voltage.
The circuits of FIGS. 1 and 2 address some of the problems of earlier prior art solutions, but also introduce problems of their own.